British drug manufacturer GlaxoSmithKline (GSK) announced that it had stopped enrollment in three phase 3 clinical trials evaluating the company’s respiratory syncytial virus (RSV) maternal vaccine candidate in pregnant women. Syncytial is pronounced sin-SISH-uhl.

The news comes after a Feb. 18 pause in these trials following a recommendation from the independent data monitoring committee. GSK says it is conducting further analyses to better understand safety data previously obtained from the three trials. This decision does not affect an ongoing phase 3 trial for the RSV vaccine in adults 60 years and older.

RSV infects the respiratory system and can have serious health repercussions, particularly in infants and older adults. At the same time, it is very common; about 90% of children will be infected during the first two years of life. Usually, an infection is experienced as a relatively mild upper respiratory tract illness with symptoms that resemble those of a common cold: a runny nose, a cough, loss of appetite.

In the United States, RSV is the most common cause of bronchiolitis and pneumonia in infants and leads to nearly 60,000 hospitalizations per year in children under 5 years of age, according to the CDC. RSV also leads to an average of 177,000 hospitalizations and 14,000 deaths per year among individuals ages 65 years and older, the CDC says.

The GSK maternal vaccine candidate contains a recombinant subunit pre-fusion RSV antigen that yielded positive results in nonpregnant women in phase 1 and 2 trials. The vaccine is intended to prevent RSV-associated lower respiratory tract infections via the transfer of maternal antibodies to newborns and infants during the first six months of life.

Currently, there are no approved RSV vaccines. Several companies are racing to change that. In addition to GSK, Pfizer, Johnson & Johnson, and Novavaxhave RSV vaccine candidates in clinical trials. In partnership with Sanofi, AstraZeneca recently gained accelerated approval from the European Medicines Agency for nirsevimab, a long-acting RSV antibody aimed to prevent lower respiratory tract infections in infants through their first RSV season.